Formation Of Nanostructures And Reprocess Properties Of Thermosets

Thermosets are a class of polymers with three-dimensional networks,which are the origins of excellent mechanical properties and solvent corrosion resistance.They have wide-spread applications in the fields such as aerospace,automobile,coatings,adhesives and electronic packaging.However,the crosslinked structures also result in some drawbacks such as large internal stress,poor fatigue resistance and low-fracture toughness.Thus,it is necessary to improve their fracture toughness.At the same time,simply burning thermosets will have an enormous negative impact on the environment.So it is urgent to find better solutions to recycling thermosets.This thesis focuses on two major issues of thermosets:Toughening and Recycling.First,a new routine for toughening epoxy thermosets with commercial triblock copolymer and ordered nanostructures of materials are both studied.Second,by preparing vitrmers with different exchange bonds and substrates,the effects of chemical structures and autocatalysis on reprocessing properties are separately studied.The research contents are as follows:(1)Chemical modifications of polystyrene-b-polybutadiene-b-polystyrene triblock copolymer and its application in thoughening epoxy thermosets with via formation of nanostructuresHydroxylated SBS was obtained via selectively reactions between 9-BBN and carbon-carbon double bonds on the side chains of SBS,and also degrees of oxidation depend on amount of 9-BBN.PS-b-PB(PCL)_0.24-b-PS was synthesized via ring opening polymerization of?-caprolactone using hydroxylated SBS as initiatior.Studies showed that ordered nanostruced epoxy thermosets were prepared via reaction-induced phase separation mechanism after epoxy thermosets were modified by incorporating toughening modifier PS-b-PB(PCL)_0.24-b-PS.At the same time,due to the“core-shell”structure,fracture toughness are significantly improved.(2)Preparation of vitrimers based on reactions between hindered urea diisocyanates and poly(hydroxyether of bisphenol A)and study on reprocessing propertiesThree kinds of hindered urea diisocyanates were synthesized via the reaction of the corresponding hindered diamines with isophorone diisocyanate(IPDI).Thermosets were prepared via the reaction between isocyanate-terminated groups and hydroxyl groups on the side chains of poly(hydroxyether of bisphenol A).Because hindered amine bonds could exchange under thermal stimulus,thermosets are endowed with reprocessable abilities.Results of reprocessing,redissolving experiments and three-point bending tests showed that thermosets containing N,N'-di-tert-butylethylenediamine and N,N'-diethylethylenediamine structures were reprocessable and piperazine-based thermosets were not reprocessable.This conclusion was further confirmed by quantitative calculation of the activation energy E_a.(3)Preparation of vitrimers based on reactions between diboronic acid and polyvinyl alcohol and study on reprocessing propertiesFirst,two diboronic acid molecules with similar structures were synthesized:nitrogen-containing and nitrogen-free.Through the reaction of diboronic acid and hydroxyl groups on the side chain of polyvinyl alcohol,thermosets were obtained.Through reprocessing,redissolving experiments and three-point bending tests,it can be seen that both boronic ester thermosets resins have reprocessing properties.Furthermore,by quantitatively calculating the activation energy via stress-relaxation experiments,it was demonstrated that the nitrogen-containing boronic ester vitrimer had smaller E_a,which meant lower energy barrier,faster bond exchange rate and a better reprocessing performance at high temperatures.